Photosensitive resin laminate and thermal processing of the same

ABSTRACT

A method of thermally developing photocurable printing blank to produce a relief pattern comprising a plurality of relief dots. The photocurable printing blank comprises a backing layer having at least one photocurable layer disposed thereon and a laser ablatable mask layer disposed on top of the at least one photocurable layer. The method includes the steps of (1) imaging the at least one photocurable layer by ablating portions of the laser ablatable mask layer; (2) laminating an oxygen barrier membrane to a top of the laser ablated mask layer; (3) exposing the printing blank to actinic radiation through the oxygen barrier membrane and mask layer to actinic radiation, thereby creating the relief pattern; (4) removing the oxygen barrier membrane; and (5) thermally developing the printing blank to remove the laser ablated mask layer and uncured portions of the photocurable layer and reveal the relief pattern. The presence of the oxygen harrier membrane produces printing dots having desired characteristics.

FIELD OF THE INVENTION

The present invention relates generally to methods of thermallyprocessing resin laminates to produce flexographic relief image printingelements for optimal printing.

BACKGROUND OF THE INVENTION

Flexographic printing plates are relief plates with image elementsraised above open areas. Generally, the plate is somewhat soft, andflexible enough to wrap around a printing cylinder, and durable enoughto print over a million copies. Such plates offer a number of advantagesto the printer, based chiefly on their durability and the case withwhich they can be made.

Flexography is commonly used for high-volume runs. Flexography isemployed for printing on a variety of substrates such as paper,paperboard stock, corrugated board, films, foils and laminates.Newspapers and grocery bags are prominent examples. Coarse surfaces andstretch films can be economically printed only by means of flexography.Corrugated board generally includes a corrugating medium which istypically a layer of pleated or multi-grooved paperboard, called“flute”, adjacent to a flat paper or paper-like layer called a “liner.”A typical corrugated board construction comprises a flute layersandwiched between two liner layers. Other embodiments may includemultiple layers of flute and/or liner. The fluted interlayer providesstructural rigidity to the corrugated board. Since corrugated board isused as packaging and formed into boxes and containers, the liner layerforming an exterior surface of the corrugated board is frequentlyprinted with identifying information for the package. The exterior linerlayer often has slight indentations due to the uneven support of theunderlying flute layer.

A problem that may be encountered when printing on corrugated boardsubstrates is the occurrence of a printing effect referred to as“fluting” (and which is also known as “banding” or “striping” or“washboarding”). Fluting may occur, when printing the liner on theexterior surface of the corrugated board, after the corrugated board hasbeen assembled. The fluting effect is visible as regions of darkprinting, i.e., bands of higher density, alternating with regions oflight printing, i.e., bands of lighter density, that correspond to theunderlying fluting structure of the corrugated board. The darkerprinting occurs where uppermost portions of the pleated innerlayerstructure support the printing surface of the liner. The fluting effectcan be apparent in areas of a printed image having tones or tint valueswhere the inked areas represent a fraction of the total area as well asin areas of the printed image where the ink coverage is more complete.This fluting effect is typically more pronounced when printing with aflexographic printing element produced using a digital workflow processbecause of the shape of the dots produced by the digital process.Furthermore, increasing the printing pressure does not eliminatefluting, and the increased pressure can cause damage to the corrugatedboard substrate. Therefore, other methods are needed to reduce flutingwhen printing on corrugated board substrates.

A typical flexographic printing plate as delivered by its manufactureris a multilayered article made of, in order, a backing, or supportlayer; one or more unexposed photocurable layers; optionally aprotective layer or slip film; and often a protective cover sheet.

The support sheet or backing layer lends support to the plate. Thesupport sheet, or backing layer, can be formed from a transparent oropaque material such as paper, cellulose film, plastic, or metal.Preferred materials include sheets made from synthetic polymericmaterials such as polyesters, polystyrene, polyolefins, polyamides, andthe like. Generally the most widely used support layer is a flexiblefilm of polyethylene terephthalate. The support sheet can optionallycomprise an adhesive layer for more secure attachment to thephotocurable layer(s). Optionally, an antihalation layer may also beprovided between the support layer and the one or more photocurablelayers. The antihalation layer is used to minimize halation caused bythe scattering of UV light within the non-image areas of thephotocurable resin layer.

The photocurable layer(s) can include any of the known photopolymers,monomers, initiators, reactive or non-reactive diluents, fillers, anddyes. The term “photocurable” refers to a composition which undergoespolymerization, cross-linking, or any other curing or hardening reactionin response to actinic radiation with the result that the unexposedportions of the material can be selectively separated and removed fromthe exposed (cured) portions to form a three-dimensional or reliefpattern of cured material. Preferred photocurable materials include anelastomeric compound, an ethylenically unsaturated compound having atleast one terminal ethylene group, and a photoinitiator. Exemplaryphotocurable materials are disclosed in European Patent Application Nos.0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No,1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No.3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos.4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S.Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz,et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No.4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., thesubject matter of each of which is herein incorporated by reference inits entirety. More than one photocurable layer may be used.

Photocurable materials generally cross-link (cure) and harden throughradical polymerization in at least some actinic wavelength region. Asused herein, actinic radiation is radiation capable of polymerizing,crosslinking or curing the photocurable layer. Actinic radiationincludes, for example, amplified (e.g., laser) and non-amplified light,particularly in the UV and violet wavelength regions. One commonly usedsource of actinic radiation is a mercury arc lamp, although othersources are generally known to those skilled in the art.

The slip film is a thin layer, which protects the photopolymer from dustand increases its ease of handling. In a conventional (“analog”) platemaking process, the slip film is transparent to UV light. In thisprocess, the printer peels the cover sheet off the printing plate blank,and places a negative on top of the slip film layer. The plate andnegative are then subjected to flood-exposure by UV light through thenegative. The areas exposed to the light cure, or harden, and theunexposed areas are removed (developed) to create the relief image onthe printing plate.

In a “digital” or “direct to plate” plate making process, a laser isguided by an image stored in an electronic data file, and is used tocreate an in situ negative in a digital (i.e., laser ablatable) maskinglayer, which is generally a slip film which has been modified to includea radiation opaque material. Portions of the laser ablatable layer arethen ablated by exposing the masking layer to laser radiation at aselected wavelength and power of the laser. Examples of laser ablatablelayers are disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang,et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subjectmatter of each of which is herein incorporated by reference in itsentirety.

After imaging, the photosensitive printing element is developed toremove the unpolymerized portions of the layer of photocurable materialand reveal the crosslinked relief image in the cured photosensitiveprinting element. Typical methods of development include washing withvarious solvents or water, often with a brush. Other possibilities fordevelopment include the use of an air knife or heat plus a blotter(i.e., thermal development). Thermal development has the advantage ofnot requiring an additional drying step after development and thusprovides the ability to go more quickly from plate to press.

Thermal development processes work by processing photopolymer printingplates using heat; the differential melting temperature between curedand uncured photopolymer is used to develop the latent image. The basicparameters of this process are known, as described in U.S. Pat. Nos.7,122,295, 6,773,859, 5,279,697, 5,175,072 and 3,264,103 and in WO01/88615, WO 01/18604, and EP 1239329, the teachings of each of whichare incorporated herein by reference in their entirety. These processesallow for the elimination of development solvents and the lengthy platedrying times needed to remove the solvent. The speed and efficiency ofthese processes allow for their use in the manufacture of flexographicplates for printing newspapers and other publications where quickturnaround times and high productivity are important.

In order for printing plates to be thermally developable, thecomposition of the photopolymer must be such that there exists asubstantial difference in the melt temperature between the cured anduncured polymer. It is precisely this difference that allows thecreation of an image in the photopolymer when heated. The uncuredphotopolymer (i.e., the portions of the photopolymer not contacted withactinic radiation) melts and/or substantially softens while the curedphotopolymer remains solid and intact at the temperature chosen. Thus,the difference in melt temperature allows the uncured photopolymer to beselectively removed, thereby creating the desired image.

Thereafter, uncured photopolymer can be softened and/or melted andremoved. In most instances, the heated printing element is contactedwith an absorbent material that absorbs or otherwise removes thesoftened and/or melted uncured photopolymer. This removal process isgenerally referred to as “blotting.”

The resulting surface, after development, has a relief pattern thatreproduces the image to be printed and which typically includes bothsolid areas and patterned areas comprising a plurality of relief dots.After the relief image is developed, the relief image printing elementmay be mounted on a press and printing commenced.

The shape of the dots and the depth of the relief, among other factors,affect the quality of the printed image. It is very difficult to printsmall graphic elements such as fine dots, lines and even text usingflexographic printing plates while maintaining open reverse text andshadows. In the lightest areas of the image (commonly referred to ashighlights) the density of the image is represented by the total area ofdots in a halftone screen representation of a continuous tone image. ForAmplitude Modulated (AM) screening, this involves shrinking a pluralityof halftone dots located on a fixed periodic grid to a very small size,the density of the highlight being represented by the area of the dots.For Frequency Modulated (FM) screening, the size of the halftone dots isgenerally maintained at some fixed value, and the number of randomly orpseudo-randomly placed dots represent the density of the image. In bothcases, it is necessary to print very small dot sizes to adequatelyrepresent the highlight areas.

Maintaining small dots on flexographic plates can be very difficult dueto the nature of the platemaking process. In digital platemakingprocesses that use a UV-opaque mask layer, the combination of the maskand UV exposure produces relief dots that have a generally conicalshape. The smallest of these dots are prone to being removed duringprocessing, which means no ink is transferred to these areas duringprinting (the dot is not “held” on plate and/or on press).Alternatively, if the dot survives processing they are susceptible todamage on press. For example small dots often fold over and/or partiallybreak off during printing causing either excess ink or no ink to betransferred.

Furthermore, photocurable resin compositions typically cure throughradical polymerization, upon exposure to actinic radiation. However, thecuring reaction can be inhibited by molecular oxygen, which is typicallydissolved in the resin compositions, because the oxygen functions as aradical scavenger. It is therefore desirable for the dissolved oxygen tobe removed from the resin composition before image-wise exposure so thatthe photocurable resin composition can be more rapidly and uniformlycured.

Various methods of removing dissolved oxygen have been developed for usein the art. For example, the removal of dissolved oxygen can beaccomplished by placing the photosensitive resin plate in an atmosphereof inert gas, such as carbon dioxide gas or nitrogen gas, beforeexposure in order to displace the dissolved oxygen. A noted drawback tothis method is that it is inconvenient and cumbersome and requires alarge space for the apparatus. In addition, as discussed in more detailbelow, this approach has not been found to be particularly effective indigital printing elements that are thermally developed.

Another approach involves subjecting the plates to a preliminaryexposure (i.e., “bump exposure”) of actinic radiation. During bumpexposure, a low intensity “pre-exposure” dose of actinic radiation isused to sensitize the resin before the plate is subjected to the higherintensity main exposure dose of actinic radiation. The bump exposure isapplied to the entire plate area and is a short, low dose exposure ofthe plate that reduces the concentration of oxygen, which inhibitsphotopolymerization of the plate (or other printing element) and aids inpreserving fine features (i.e., highlight dots, fine lines, isolateddots, etc.) on the finished plate. However, the pre-sensitization stepcan also cause shadow tones to fill in, thereby reducing the tonal rangeof the halftones in the image.

The bump exposure requires specific conditions that are limited to onlyquench the dissolved oxygen, such as exposing time, irradiated lightintensity and the like. In addition, a selective preliminary exposure,as discussed for example in U.S. Pat. Publication No. 2009/0043138 toRoberts et al., the subject matter of which is herein incorporated byreference in its entirety, has been proposed.

Other efforts have involved special plate formulations alone or incombination with the bump exposure, such as in U.S. Pat. No. 5,330,882to Kawaguchi, the subject matter of which is herein, incorporated byreference in its entirety, which suggests the use of a separate dye thatis added to the resin to absorb actinic radiation at wavelengths atleast 100 nm removed from the wavelengths absorbed by the mainphotoinitiator. U.S. Pat. No. 4,540,649 to Sakurai, incorporated hereinby reference in its entirety, describes a photopolymerizable compositionthat contains at least one water soluble polymer, a photopolymerizationinitiator and a condensation reaction product of N-methylol acrylamide,N-methylol methacrylamide, N-alkyloxymethyl acrylamide orN-alkyloxymethyl methacrylamide and a melamine derivative, which,according to the inventors, eliminates the need for pre-exposureconditioning and produces a chemically and thermally stable plate.

However all of these methods are still deficient in producing a reliefimage printing element having a superior dot structure, especially whendesigned for printing corrugated board substrates. In addition, all ofthe methods described above have also not been shown to produce a reliefimage printing element having superior dot structure when the reliefimage is subjected to a thermal development step.

When developing in solvent, the main consideration is whether or not thesolvent can swell and disperse/dissolves the uncured photopolymer andassociated barrier layers, in combination with the appropriatemechanical agitation, resulting in a clean printing plate free ofcontaminants, surface defects, or other unwanted solvent-basedphenomenon common to the platemaking industry.

In contrast, developing plates thermally sometimes requires otherconsiderations. Digital plates, when exposed by conventional means(i.e., in air) have previously been believed to be interchangeablewhether subject to a solvent development process or a thermaldevelopment process, using the same base resin formulation. Analogthermal has shown itself to be more challenging, oftentimes requiringthe use of a novel slip film or unique properties to the resin itself,such as very high melt flow.

Thus, there is a need for an improved process for preparing relief imageprinting elements that are subjected to a thermal development process.

There is also a need for an improved relief image printing element thatcomprises an improved relief structure including printing dots that areconfigured for superior printing performance on various substrates.

The present invention relates generally to a digital plate with dots ofa controlled architecture beneficial to printing (i.e., flat top, steepshoulders), capable of being thermally processed with no detriment inplate quality or print performance.

The present invention also provides a means for exposing and processingan analog plate via the same exposure technique, which can also beprocessed thermally with no detriment to material quality or printperformance.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodof thermally developing digital relief image printing elements.

It is another object of the present invention to provide an improvedmethod of thermally developing analog relief image printing elements.

It is another object of the present invention to provide an improvedmethod of thermally developing relief image printing plate that producesprinting dots having a flat top and steep shoulders.

It is still another object of the present invention to provide a methodof imaging and developing relief image printing elements that provides agood result when printing on corrugated board substrates.

It is another object of the present invention to produce a relief imageprinting plate that reduces print fluting when printing on corrugatedboard substrates.

It is another object of the present invention to create a relief imageprinting element that comprises printing dots having a superior dotstructure in terms of print surface, edge definition, shoulder angle,depth and dot height.

It is another object of the present invention to provide a dot shape andstructure on the printing element that is highly resistant to printfluting.

It is still another object of the present invention to control thesurface roughness of the print surface of the relief image printingelement.

The inventors here have discovered that a characteristic of platesprocessed by thermal means is higher surface roughness of both solidareas and the tops of dots, as well as the floor of the plate. This isdue to the fact that ‘blotting’ is incapable of removing all of thephotopolymer during thermal processing. There is always some smallamount of residual polymer left on the plate, both on the reliefelements and on the plate's floor. The texture of the blotter materialis typically transferred into this remnant photopolymer. In the floorareas of the plate, this distinctive pattern has only cosmetic effect.However, on the relief elements, this texture can be problematic. If theroughness of the texture is excessive, it can affect print quality byactually transferring the pattern to the surface being printed,resulting in qualitative print defects often described as mottling orpinholing, and the quantitative print defect of reduced solid inkdensity (SID). These defects generally degrade the quality of theprinted articles made from plates with excessive roughness, reducing thevibrancy of colors and making it difficult to achieve consistent colorreproduction.

Some degree of plate surface roughness can be beneficial to printperformance, but excessive surface roughness can have theabove-described negative effects. The definition of ‘excessive’ platesurface roughness varies depending upon many factors, including thesubstrate printed, the ink characteristics, and the amount of ink usedon each image. Generally, the inventor have found that plate surfaceroughness of less than 2000 nm (R_(a)) is required to achieve good anduniform solid ink coverage, with plate surface roughness of less than1200 nm preferred, and plate surface roughness of less than 800 nm mostpreferred.

To these ends, in a preferred embodiment, the present invention relatesgenerally to a method of thermally developing a photocurable printingblank to produce a relief pattern comprising a plurality of relief dots,wherein the photocurable printing blank comprises a backing layer havingat least one photocurable layer disposed thereon and a laser ablatablemask layer disposed on top of the at least one photocurable layer, themethod comprising the steps of:

-   -   a) imaging the at least one photocurable layer by selectively        ablating the laser ablatable mask layer to create an image on        the surface of the at least one photocurable layer;    -   b) laminating an oxygen barrier membrane to a top of the laser        ablated mask layer;    -   c) exposing the printing blank to actinic radiation through the        oxygen barrier membrane and mask layer to one or more sources of        actinic radiation to selectively crosslink and cure portions of        the at least one photocurable layer, wherein the at least one        photocurable layer is crosslinked and cured in the portions not        covered by the mask layer, thereby creating the relief pattern;    -   d) removing the oxygen barrier membrane from the top of the        laser ablated mask layer; and    -   e) thermally developing the printing blank remove the laser        ablated mask layer and uncured portions of the photocurable        layer and reveal the relief pattern;

wherein the presence of the oxygen barrier membrane during the exposurestep produces printing dots having desired characteristics.

In another preferred embodiment, the present invention relates generallyto a method of thermally developing a photocurable printing blank toproduce a relief pattern comprising a plurality of relief dots, whereinthe photocurable printing blank comprises a backing layer having atleast one photocurable layer disposed thereon, the method comprising thesteps of:

-   -   a) laminating an oxygen barrier membrane to a top of the        photocurable layer;    -   b) placing a negative of a desired relief image on top of the        oxygen barrier membrane;    -   c) exposing the printing blank to actinic radiation through the        oxygen barrier membrane and negative to selectively crosslink        and cure the at least one photocurable layer, wherein the at        least one photocurable layer is crosslinked and cured in the        areas that are not covered by the negative, thereby creating the        desired relief image;    -   d) removing the oxygen barrier membrane and the negative from        the top of the at least one photocurable layer; and    -   e) thermally developing the printing blank to remove uncured        portions of the photocurable layer and reveal the desired relief        image;

wherein the presence of the oxygen barrier membrane during the exposurestep results in the plurality of printing dots having desiredcharacteristics.

The negative can also act as an oxygen barrier itself. If this is thecase, then the negative should be laminated to the at least onephotocurable layer and the separate oxygen barrier membrane is notrequired. In this case the desired image can be created by ink jetting aradiation opaque material onto the oxygen barrier so that it can serveas a negative also.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to thefollowing description taken in connection with the accompanying figures,in which:

FIG. 1 depicts a graph of surface roughness values of digital platesexposed and processed via various techniques.

FIGS. 2A, 2B and 2C depict a comparison of thermally processed platedots at 5%, 20% and 50% for conventional thermal, laminated thermal andnitrogen exposed thermal developing processes.

FIGS. 3A, 3B and 3C depict a comparison of thermally processed platelines and reverses for conventional thermal, laminated thermal andnitrogen exposed thermal development processes.

FIGS. 4A, 4B and 4C depict a comparison of text from thermally processedplates for conventional thermal, laminated thermal and nitrogen exposedthermal development processes,

FIGS. 5A, 5B and 5C depict a comparison of print quality from thermallyprocessed plates for conventional thermal, laminated thermal andnitrogen exposed thermal development processes.

FIG. 6 depicts a view of the clean out achieved on an analog plate usingthe laminated thermal development process of the instant invention.

FIG. 7 depicts a schematic representation of four dot shape measurementsrelated to the creation of an optimum dot for flexographic printing.

FIG. 8 depicts the measurement of the dot shoulder angle θ.

FIG. 9 depicts relief image definitions.

FIG. 10 depicts a means of characterizing the planarity of a dot'sprinting surface where p is the distance across the dot top and r_(t) isthe radius of curvature across the surface of the dot.

FIG. 11 depicts a flexo dot and its edge, where p is the distance acrossthe dot top. This is used in the characterization of edge sharpness,r_(e):p, where r_(e) is the radius of curvature at the intersection ofthe shoulder and the top of the dot.

DETAILED DESCRIPTION OF THE INVENTION

The inventors of the present invention have found that the shape andstructure of a printing dot has a profound impact on the way it prints.This is especially true in digital relief image printing elements. Theinventors of the present invention have also determined that therespecial considerations which must be addressed when using thermaldevelopment processes to provide a relief surface that includes reliefprinting dots having flat tops and steep shoulders.

The inventors of the present invention have discovered that there is anadvantage to reducing the impact of oxygen inhibition during plateexposure while simultaneously maintaining the physical propertiesnecessary to produce high quality thermally processed printing plates.

The present invention relates generally to the lamination of a barriermembrane onto the surface of an ablated digital plate or an uncoatedplate that can be imaged via an analog method. The subsequent plate isthen thermally processed to remove uncured photopolymer, therebyproducing a relief printing plate. The membrane's function is to serveas a oxygen barrier which allows for altering of the shape of the formeddots on the printing plate. The result of the use of this barrier layeris the advantageous control of the curing mechanism such that thefollowing occurs:

-   -   1) Dots are formed without the restricting effect of oxygen        inhibition, resulting in flat tops and steep shoulder angles;    -   2) The curing rate is controlled to the point that optimum        reverse depths are maintained and should angles are not        excessively broadened;    -   3) The resulting membrane lamination minimizes the creation of        excessive surface roughness during thermal processing; and    -   4) The resulting membrane allows for more efficient thermal        processing of an analog printing form than currently existing        analog plate constructions, since the membrane is removed prior        to processing.

The present invention utilizes the aforementioned advantages of thelaminated membrane as an oxygen barrier and combines them with thesurprising discovery that laminated thermally processed plates performbetter in print studies than standard thermally processed plates as wellas those exposed in inert gas media, showing reduced dot gain andcleaner solids and reverse print.

In a preferred embodiment, the present invention relates generally to amethod of thermally developing a photocurable printing blank to producea relief pattern comprising a plurality of relief dots, wherein thephotocurable printing blank comprises a backing layer having at leastone photocurable layer disposed thereon and a laser ablatable mask layerdisposed on top of the at least one photocurable layer, the methodcomprising the steps of:

-   -   a) imaging the at least one photocurable layer by selectively        ablating the laser ablatable mask layer to create an image on        the surface of the at least one photocurable layer;    -   b) laminating an oxygen barrier membrane to a top of the laser        ablated mask layer;    -   c) exposing the at least one photocurable layer to actinic        radiation through the oxygen barrier membrane and mask layer to        one or more sources of actinic radiation to selectively        crosslink and cure portions of the at least one photocurable        layer, wherein the at least one photocurable layer is        crosslinked and cured in the portions not covered by the mask        layer, thereby creating the relief pattern;    -   d) removing the oxygen barrier membrane from the top of the        laser ablated mask layer; and    -   e) thermally developing the printing blank remove the laser        ablated mask layer and uncured portions of the photocurable        layer and reveal the relief pattern;

wherein the presence of the oxygen barrier membrane produces printingdots having desired geometric parameters.

The desired geometric parameters of the printing dots are typically oneor more of steep shoulder angles, planarity of the dot surface,sufficient depth of relief between the dots, sharpness of the edge atthe point where the dot top transitions to the dot shoulder, low surfaceroughness and combinations thereof. One can manipulate the resultantshape of the printing dots to optimize printing by utilizing the methodsdescribed herein.

The inventors of the present invention have found that a particular setof geometric characteristics define a flexo dot shape that yieldssuperior printing performance, as shown in FIG. 7. These geometricparameters include, but are not limited to, (1) planarity of the dotsurface; (2) shoulder angle of the dot; (3) depth of relief between thedots; and (4) sharpness of the edge at the point where the dot toptransitions to the dot shoulder. These geometric parameters aredescribed in more detail in related patent application Ser. No.12/571,523 to Recchia and Ser. No. 12/660,451 to Recchia et al., thesubject matter of each of which is herein incorporated by reference inits entirety. However the particular use of these geometric parametersin optimizing print quality of printing dots produced in thermallydevelopment processes has not been previously been investigated.

Firstly, the angle of the dot shoulder has been found to be a goodpredictor of print performance. The dot shoulder is defined as shown inFIG. 8 as the angle θ formed by the dot's top and side. At the extreme,a vertical column would have a 90° shoulder angle, but in practice mostflexo dots have an angle that is considerable lower, often nearer 45°than 90°.

The shoulder angle can vary depending on the size of the dots as well.Small dots, for example in the 1-15% range, may have large shoulderangles, while larger dots, for example greater than about 15% dots mayexhibit smaller shoulder angles. It is desirable for all dots to havethe largest shoulder angle possible. In one embodiment, the desiredcharacteristics comprise steep shoulder angles and the shoulder angle ofeach of the plurality of dots is such that the overall shoulder angle isgreater than about 50°, preferably greater than about 70°.

There are two competing geometric constraints on shoulder angle—dotstability and impression sensitivity. A large shoulder angle minimizesimpression sensitivity and gives the widest operating window on press,but at the expense of dot stability and durability. In contrast, a lowershoulder angle improves dot stability but makes the dot more sensitiveto impression on press. As used herein, dot shoulder angle means theangle formed by the intersection of a horizontal line tangential to thetop of the dot and a line representing the adjacent dot side wall.

In another embodiment, the desired characteristics comprise planarity ofthe dot surface. The planarity of the top of a dot can be measured asthe radius of curvature across the top surface of the dots, r_(t), asshown in FIG. 10. Preferably, the top surface of the dot has aplanarity, where the radius of curvature of the dot top is greater thanthe total thickness of the at least one layer of photocurable material,more preferably twice the thickness of the at least one layer ofphotocurable material, and most preferably, more than three times thetotal thickness of the photopolymer layer. A planar dot surface ispreferred throughout the tonal range. Most preferred are planar dotsurfaces, even on dots in the highlight range (i.e., 0-10% tonal).

In still another embodiment, the desired characteristic of the printingdots is low surface roughness and the surface roughness of the top ofthe plurality of relief printing dots is less than about 2000 nm,preferably less than about 1250 nm, and most preferably less than 800nm.

In another embodiment, the desired characteristic of the printing dotsis sufficient depth of relief between the dots, and a dot relief ofprinting element is greater than about 9% of the overall plate relief,preferably greater than about 12% of the overall plate relief. Platerelief is expressed as the distance between the floor of the plate andthe top of a solid relief surface, as shown in FIG. 9. For example, a0.125 inch thick plate is typically made so as to have an 0.040 inchrelief. However, the plate relief is typically much larger than therelief between dots in tone patches (i.e., the “dot relief”), which is aresult of the close spacing of the dots in tonal areas. The low reliefbetween dots in tonal areas means that the dots are structurallywell-supported, but can cause problems during printing as ink builds upon the plate and eventually fills in the areas between dots, causing dotbridging or dirty print. The inventors have found that deeper dot reliefcan reduce this problem significantly, leading to longer print runs withless operator interference, a capability that is often called “cleanerprinting.”

In another embodiment, the desired characteristic is sharpness of theedge at the point where the dot top transitions to the dot shoulder. Itis generally preferred that the dot edges be sharp and defined. Thesewell-defined dot edges better separate the “printing” portion from the“support” portion of the dot, allowing for a more consistent contactarea between the dot and the substrate during printing. Edge sharpnesscan be defined as the ratio of r_(e), the radius of curvature (at theintersection of the shoulder and the top of the dot) to p, the width ofthe dot's top or printing surface, as shown in FIG. 11. For a trulyround-tipped dot, it is difficult to define the exact printing surfacebecause there is not really an edge in the commonly understood sense,and the ratio of r_(e):p can approach 50%. In contrast, a sharp-edgeddot would have a very small value of r_(e), and r_(e):p would approachzero. In practice, an r_(e):p of less than 5% is preferred, with anr_(e):p of less than 2% being most preferred.

A wide range of materials can serve as the barrier membrane layer. Threequalities that the inventors have identified in producing effectivebarrier layers include optical transparency, low thickness and oxygentransport inhibition. Oxygen transport inhibition is measured in termsof a low oxygen diffusion coefficient. As noted, the oxygen diffusioncoefficient of the oxygen barrier membrane is typically less than about6.9×10⁻⁹ m²/sec, more preferably less than about 6.9×10⁻¹⁰ m²/sec, andmost preferably less than about 6.9×10⁻¹¹ m²/sec.

For thermal processing, the most preferred oxygen barrier membranes areclear films that minimize light scattering. Examples of materials whichare suitable for use as the barrier membrane layer include polyamides,polyvinyl alcohol, hydroxyalkyl cellulose, polyvinyl pyrrolidinone,copolymers of ethylene and vinyl acetate, amphoteric interpolymers,cellulose acetate butyrate, alkyl cellulose, butryal, cyclic rubbers,and combinations of one or more of the foregoing. In addition, filmssuch as polypropylene, polyethylene, polyvinyl chloride, polyester andsimilar clear films can also serve well as barrier films. In onepreferred embodiment, the barrier membrane layer comprises apolypropylene film or a polyethylene terephthalate film. Oneparticularly preferred barrier membrane is a Fuji® Final Proof receiversheet membrane available from Fuji Films.

The barrier membrane should be as thin as possible, consistent with thestructural needs for handling of the film and the film/photopolymerplate combination. Barrier membrane thicknesses between about 1 and 100microns are preferred, with thickness of between about 1 and about 20microns being most preferred.

The barrier membrane needs to have a sufficient optical transparency sothat the membrane will not detrimentally absorb or deflect the actinicradiation used to expose the photosensitive printing blank. As such itis preferable that the barrier membrane have an optical transparency ofat least 50%, most preferably at least 75%.

The barrier membrane needs to be sufficiently impermeable to oxygendiffusion so that it can effectively limit diffusion of oxygen into thephotocurable layer during exposure to actinic radiation. The inventorsherein have determined that the barrier membrane materials noted abovein the thicknesses noted above will substantially limit the diffusion ofoxygen into the photocurable layer when used as described herein.

In another embodiment of the present invention, the barrier membranecomprises a smooth nanotechnology film with a roughness of less than 100nm. In this embodiment, the average surface roughness of the printingplate can be controlled to less than about 100 nm.

The barrier layer may be laminated to the surface of the printing plateusing pressure and/or heat in a typical lamination process.

Suitable thermal development processes are generally well known to thoseskilled in the art. In one embodiment, the thermal development stepcomprises the steps of:

-   -   a) softening non-crosslinked polymer on the imaged and exposed        surface of the printing element by contacting the imaged and        exposed surface with an absorbent layer capable of absorbing        non-crosslinked portions of the at least one layer of        photocurable material when it has been heated to a temperature        of between 40° and 200° C.,    -   b) heating said at least one layer of photocurable material to a        temperature of between 40° and 200° C. and allowing the        non-crosslinked portions of the at least one layer of        photocurable material in contact with the absorbent layer to be        absorbed by said absorbent layer, and    -   c) removing said absorbent layer containing the non-crosslinked        portion of the at least one photocurable layer, whereby the        relief pattern is revealed.

In addition, the barrier layer method can be used in an analogconstruction wherein a barrier layer is laminated to a photopolymerresin containing non infrared UV absorbing slip film layer. A negativeis then placed upon the barrier layer, and the platemaking occurs viastandard analog platemaking practices. FIG. 6 shows the clean-outachieved using this method.

More specifically, in another preferred embodiment, the presentinvention relates generally to a method of thermally developing aphotocurable printing blank to produce a relief pattern comprising aplurality of relief dots, wherein the photocurable printing blankcomprises a backing layer having at least one photocurable layerdisposed thereon, the method comprising the steps of:

-   -   a) laminating an oxygen barrier membrane to a top of the        photocurable layer;    -   b) placing a negative of a desired relief image on top of the        oxygen barrier membrane;    -   c) exposing the at least one photocurable layer to actinic        radiation through the oxygen barrier membrane and negative to        selectively crosslink and cure the at least one photocurable        layer, wherein the at least one photocurable layer is        crosslinked and cured in the areas that are not covered by the        negative, thereby creating the desired relief image;    -   d) removing the oxygen barrier membrane and the negative from        the top of the at least one photocurable layer; and    -   e) thermally developing the printing blank to remove uncured        portions of the photocurable layer and reveal the desired relief        image;

wherein the presence of the oxygen barrier membrane results in theplurality of printing dots having desired characteristics.

The negative can also act as an oxygen barrier itself. If this is thecase, then the negative should be laminated to the at least onephotocurable layer and the separate oxygen barrier membrane is notrequired. In this case the desired image can be created by ink jetting aradiation opaque material onto the oxygen barrier so that it can serveas a negative also.

To that end, in another embodiment, the present invention relatesgenerally to a method of thermally developing a photocurable printingblank to produce a relief pattern comprising plurality of relief dots,wherein the photocurable printing blank comprises a backing layer havingat least one photocurable layer disposed thereon, the method comprisingthe step of

-   -   a) laminating a negative of a desired relief image on top of the        at least one layer of photocurable material;    -   b) exposing the printing blank to actinic radiation through the        negative to selectively crosslink and cure the at least one        photocurable layer, wherein the at least one photocurable layer        is crosslinked and cured in the areas that are not covered by        the negative, thereby creating the desired relief image;    -   c) removing the negative from the top of the at least one layer        of photocurable material; and    -   d) thermally developing the printing blank to remove uncured        portions of the photocurable layer and reveal the desired relief        image,

wherein the negative serves as a oxygen barrier, which allows foraltering of the shape of the formed relief dots.

FIG. 1 depicts a graph of the levels of surface roughness of digitalplates that were exposed and processed by the indicated means. As can beseen from FIG. 1, the numbers change with process type, with thermalprocessing typically giving higher surface roughness values. In fact,the membrane lamination can reduce the surface roughness in the case ofsolvent processed materials as well, depending upon the product.Surprisingly, the oxygen-free environment provided by nitrogen does notimprove the surface cure to a level that is impervious to theroughening/embossing effect that comes about from the impression of thenon-woven media into the polymer surface at elevated temperatures. Infact, the roughness is noticeably increased when the plate is exposedunder inert gas and then thermally processed.

Shown in FIGS. 2A to 2C are dots and reverses of thermally developedplates. When compared with the nitrogen exposed plates shown in FIG. 2C,the reverses of the laminated processed plates shown in FIG. 2B areclearly deeper, the dot angles steeper, and the surface smoother than inthe nitrogen exposed plate.

Table 1 shows the reverse depths of thermally processed plate materialsexposed via lamination and inert gas techniques for 10 mil reverse, 15mil reverse and 30 mil reverse.

TABLE 1 Reverse Depths of Thermally Processed Plate Materials ExposedVia Lamination and Inert Gas Techniques Plate Material 10 mil reverse 15mil reverse 30 mil reverse Max, laminated 3.55 4.35 9.00 Max, nitrogen2.25 2.55 5.30 Rave, laminated 4.90 6.40 11.55 Rave, nitrogen 3.25 2.103.95 CST, laminated 4.65 5.65 10.50 CST, nitrogen 2.95 4.00 7.85

Shown in FIGS. 3A, 3B and 3C is a comparison of thermally processedplate lines and reverses for conventional thermal, laminated thermal andnitrogen exposed thermal development processes and shown in FIGS. 4A, 4Band 4C is a comparison of text from thermally processed plates forconventional thermal, laminated thermal and nitrogen exposed thermaldevelopment processes.

The resulting clean-out also impacts the final print performance. Shownin FIGS. 5A, 5B and 5C are comparisons of conventional thermal,laminated thermal, and nitrogen thermal plates, showing the cleardifference in text sharpness and clarity.

Finally, once the plates have been subjected to thermal development, therelief image printing element is mounted on a printing cylinder of aprinting press and printing is commenced.

Thus, it can be seen that the method of making the relief image printingelement described herein produces a relief image printing element havinga relief pattern comprising relief dots to be printed that areconfigured for optimal print performance. In addition, through thelamination method described herein, it is possible to make thermallydeveloped plates, both digital and analog that have optimized geometriccharacteristics of the relief dots in the resulting relief image toproduce a desired result.

What is claimed is:
 1. A method of thermally developing a photocurableprinting blank to produce a relief pattern comprising a plurality ofprinting dots, wherein the photocurable printing blank comprises abacking layer having at least one photocurable layer disposed thereonand a laser ablatable mask layer disposed on top of the at least onephotocurable layer, the method comprising the step of: a) imaging the atleast one photocurable layer by selectively ablating the laser ablatablemask layer to create an image on the surface of the at least onephotocurable layer; b) laminating an oxygen barrier membrane to a top ofthe laser ablated mask layer; c) exposing the printing blank to actinicradiation through the oxygen barrier membrane and laser ablatable masklayer to one or more sources of actinic radiation to selectivelycrosslink and cure portions of the at least one photocurable layer,wherein the at least one photocurable layer is crosslinked and cured inthe portions not covered by the mask layer, thereby creating the reliefpattern; d) removing the oxygen barrier membrane from the top of thelaser ablated mask layer; and e) thermally developing the photocurableprinting blank to remove the laser ablated mask layer and uncuredportions of the at least one photocurable layer and reveal the reliefpattern.
 2. The method according to claim 1, wherein the printing dotshave one or more characteristics selected from: a) steep shoulderangles, wherein the shoulder angle of each of the printing dots is suchthat the overall shoulder angle is greater than about 50°; b) planarityof the printing dot surface, wherein the planarity of a top surface ofthe printing dots is such that the radius of curvature of the topsurface of the printing dots, r_(t), is greater than the total thicknessof the at least one layer of photocurable material; c) sufficient depthof relief between the printing dots, wherein relief between the printingdots is greater than about 9% of overall relief; d) sharpness of theedge at the point where the printing dot top transitions to the printingdot shoulder, wherein a ratio of r_(e):p is less than 5%; and e) lowsurface roughness, wherein the surface roughness of the top of theprinting dots is less than about 700 nm.
 3. The method according toclaim 2, wherein the at least one characteristic comprises steepshoulder angles, wherein the shoulder angle of each of the printing dotsis such that the overall shoulder angle is greater than about 50°. 4.The method according to claim 3, wherein the shoulder angle of each ofthe plurality of printing dots is such that overall shoulder angle isgreater than about 70°.
 5. The method according to claim 2, wherein theat least one characteristic comprises planarity of the printing dotsurface, wherein the planarity of a top surface of the printing dots issuch that the radius of curvature of the top surface of the printingdots r₊, is greater than the thickness of the at least one layer ofphotocurable material.
 6. The method according to claim 2, wherein theat least one characteristic comprises low surface roughness, wherein thesurface roughness of the top of the printing dots is less than about 700nm.
 7. The method according to claim 6, wherein the surface roughness ofthe top of the plurality of printing dots is less than about 800 nm. 8.The method according to claim 2, wherein relief is greater than about12% of the overall relief.
 9. The method according to claim 2, whereinthe at least one characteristic comprises edge sharpness of the printingdots at the point where the printing dot top transitions to the printingdot shoulder, wherein a ratio of r_(e):p is less than 5%.
 10. The methodaccording to claim 9, wherein the ratio of r_(e):p is less than 2%. 11.The method according to claim 1, wherein the oxygen barrier membrane isselected from the group consisting of polyamides, polyvinyl alcohol,hydroxyalkyl cellulose, polyvinyl pyrrolidinone, copolymers of ethyleneand vinyl acetate, amphoteric interpolymers, cellulose acetate butyrate,alkyl cellulose, butryal, cyclic rubbers, and combinations of one ormore of the foregoing.
 12. The method according to claim 1, wherein theoxygen barrier membrane comprises a clear film selected from the groupconsisting of polypropylene, polyethylene, polyvinyl chloride, polyesterand combinations of one or more of the foregoing.
 13. The methodaccording to claim 12, wherein the oxygen barrier membrane comprises apolypropylene film or a polyethylene terephthalate film.
 14. The methodaccording to claim 1, wherein the oxygen barrier membrane has athickness of between about 1 and 100 microns.
 15. The method accordingto claim 14, wherein the oxygen barrier membrane has a thickness ofbetween about 1 and about 20 microns.
 16. The method according to claim1, wherein the oxygen barrier membrane has an optical transparency of atleast about 50%.
 17. The method according to claim 16, wherein theoxygen barrier membrane has an optical transparency of at least about75%.
 18. The method according to claim 1, wherein the oxygen barriermembrane has an oxygen diffusion coefficient of less than 6.9×10⁻⁹m²/sec.
 19. The method according to claim 18, wherein the oxygen barriermembrane has an oxygen diffusion coefficient of less than 6.9×10⁻¹⁰m²/sec.
 20. The method according to claim 19, wherein the oxygen barriermembrane has an oxygen diffusion coefficient of less than 6.9×10⁻¹¹m²/sec.
 21. The method according to claim 1, wherein the step ofthermally developing the printing blank comprises: a) softeningnon-crosslinked polymer on the imaged and exposed surface of theprinting blank by contacting the imaged and exposed surface with anabsorbent layer capable of absorbing non-crosslinked portions of the atleast one layer of photocurable material when it has been heated to atemperature of between 40° and 200° C., b) heating said at least onelayer of photocurable material to a temperature of between 40° and 200°C. and allowing the non-crosslinked portions of the at least one layerof photocurable material in contact with the absorbent layer to beabsorbed by said absorbent layer, and c) removing said absorbent layercontaining the non-crosslinked portion of the at least one photocurablelayer, whereby the relief pattern is revealed.
 22. The method accordingto claim 1, wherein the printing blank is not imaged in an inertenvironment.
 23. The method according to claim 1, wherein the printingdots have one or more characteristics selected from: a) steep shoulderangles, wherein the shoulder angle of each of the printing dots is suchthat the overall shoulder angle is greater than about 50° C.; b)planarity of the printing dot surface, wherein the planarity of a topsurface of the printing dots is such that the radius of curvature of thetop surface of the printing dots, r_(t), is greater than the totalthickness of the at least one layer of photocurable material; c)sufficient depth of relief between the printing dots, wherein reliefbetween the printing dots is greater than about 9% of the overallrelief; d) sharpness of the edge at the point where the printing dot toptransitions to the printing dot shoulder, wherein a ratio of r_(e):p isless than 5%; and e) low surface roughness, wherein the surfaceroughness of the top of the printing dots is less than about 800 nm.